The present invention relates to a method and apparatus for improving the impaired hearing of a human subject using an implantable hearing aid device. More specifically, the present invention relates to a method and apparatus for improving the performance of implantable hearing aid devices.
The human hearing mechanism is a complex system of levers, membranes, fluid reservoirs, neurons, and hair cells which must work in concert to deliver nervous stimuli to the brain where this information is compiled into what we perceive as sound. Because the human hearing system encompasses a complicated mix of acoustic, mechanical, and neurological systems, there is ample opportunity for its impairment. Unfortunately, this is often the case.
Attempts to remedy such deficiencies have a long history. The first electronic hearing aids began making their debut in the early 1900's. The development of the transistor led to smaller, more power-efficient aids that began to appear in the 1950's. In the 1960's and 70's, the hearing aid enjoyed a period of accelerated development.
The hearing impaired patient now has a wide variety of hearing devices to choose from. Devices having improved circuits, now permit a hearing aid's frequency response to be customized to a patient's individual hearing loss. New devices located completely in the patient's ear canal are available that are cosmetically superior to the large, bulky devices of years past.
A number of auditory system defects are now known to impair or prevent hearing. To illustrate such defects, a schematic representation of part of the human auditory system is shown in FIG. 1. The auditory system is generally comprised of an external ear AA, a middle ear JJ, and an internal ear FF. External ear AA includes ear canal BB and tympanic membrane CC, and internal ear FF includes an oval window EE and a vestibule GG (a passageway to the cochlea (not shown)). Middle ear JJ is positioned between external ear AA and internal ear FF, and includes eustachian tube KK and three bones called ossicles DD. Ossicles DD include a malleus LL, an incus MM, and a stapes HH, which are positioned between and connected to tympanic membrane CC and oval window EE.
In a person with normal hearing, sound enters the external ear AA where it is slightly amplified by the resonant characteristics of ear canal BB. The sound waves produce vibrations in tympanic membrane CC, the part of external ear AA that is positioned at the distal end of ear canal BB. The force of these vibrations is magnified by ossicles DD.
Upon vibration of ossicles DD, oval window EE, which is part of internal ear FF, conducts the vibrations to cochlear fluid (not shown) in inner ear FF thereby stimulating receptor cells, or hairs, within the cochlea (not shown). Vibrations in the cochlear fluid also conduct vibrations to the round window (not shown). In response to the stimulation, the hairs generate an electrochemical signal which is delivered to the brain via one of the cranial nerves, causing the brain to perceive sound.
The vibratory structures of the ear include the tympanic membrane, ossicles (malleus, incus, and stapes), oval window, round window, and cochlea. Each of the vibratory structures of the ear vibrates to some degree when a person with normal hearing hears sound waves. However, hearing loss in a person may be evidenced by one or more vibratory structures vibrating less than normal or not at all. Some patients with hearing loss have ossicles that lack the resiliency necessary to increase the force of vibrations to a level that will adequately stimulate the receptor cells in the cochlea. Other patients have ossicles that are broken, and which therefore do not conduct sound vibrations to the oval window.
Various types of hearing aids have been developed to restore or improve hearing for the hearing impaired. With conventional hearing aids, sound is detected by a microphone, amplified using amplification circuitry, and transmitted in the form of acoustical energy by a speaker or another type of transducer into the middle ear by way of the tympanic membrane. Often, the acoustical energy delivered by the speaker is detected by the microphone, causing a high-pitched feedback whistle. Moreover, the amplified sound produced by conventional hearing aids normally includes a significant amount of distortion.
An interesting implementation concerns implantable hearing aids configured for disposition principally within the middle ear space. For example, such an approach could provide a transducer capable of converting mechanical vibration within the ossicular chain into an output voltage (e.g., a piezoelectric transducer). That output voltage could be converted to mechanical vibrations (e.g., again, by a piezoelectric transducer) and applied to the area of the oval window to stimulate it. Alternatively, the output voltage could be used to electrically stimulate the auditory nerve. As an alternative, the stapes may be removed and the hearing aid physically located in its stead, conditions permitting. Under circumstances where the stapes is removed, the end of the incus is free-standing and the hearing aid may be physically associated with it, such as by means of crimpable rings or the like. Thus the hearing aid serves as an integral part of the mechanical linkage in the transmission of forces from the eardrum to the oval window in all events, whether or not the integrity or continuity of the ossicular chain remains unimpaired. That being the case, however, unwanted mechanical feedback through the ossicular chain is a possibility, diminishing the overall efficacy of this approach.
Methods have been devised to avoid this unwanted feedback, and so improve the hearing of patients using such devices. For example, such an invention is described in "IMPLANTABLE HEARING AID AND METHOD OF IMPROVING HEARING" by D. W. Schaefer (U.S. Pat. No. 4,729,366), which is hereby incorporated by reference in its entirety. Schaefer describes a method and apparatus for improving the impaired hearing of a subject utilizing a device which receives vibrations from one of the subject's ossicles or the subject's eardrum, and provides an amplified version of those vibrations to the subject's inner ear.
Referring to FIG. 2, one embodiment of an implantable hearing device 10 according to Schaefer is shown disposed in an antrum 12 surgically developed in the mastoid bone of a subject's skull 14. Antrum 12 communicates with a middle ear space 16 of the subject. Device 10 may include, for example, a power source 18, an amplifier 20, an input transducer 22, and an output transducer 24. Input transducer 22 converts mechanical vibrations to electrical signals, while output transducer 24 converts electrical signals to mechanical vibrations.
Input transducer 22 and output transducer 24 may, for example, each include a piezoelectric element cooperating with a resilient diaphragm. A connecting member 25, mounted on a diaphragm (not shown), is operatively coupled to piezoelectric element (also not shown). Schaefer indicates that connecting member 25 is preferably a rigid stainless steel wire. (The respective connecting members 25 associated with input transducer 22 and output transducer 24 will be referenced as wires 25A and 25B, respectively.)
Input transducer 22 converts vibrations of a tympanic membrane 26 into electrical signals. Input transducer 22 is mechanically coupled to tympanic membrane 26 by wire 25A to a malleus 30 of the subject. Wire 25A must have an acceptable degree of stiffness and may be affixed to malleus 30 utilizing surgical techniques similar to those used in ossicular reconstructive surgery, for example. Tympanic membrane 26 vibrates in response to sound waves 32. The vibrations are transmitted to malleus 30, through wire 25A to input transducer 22. These vibrations are converted to electrical signals by a first piezoelectric element within input transducer 22 (not shown). The electrical signals are then applied to amplifier 20. Amplifier 20 amplifies these input signals in a manner sufficient to drive output transducer 24, compensating for deficiencies in the frequency response of the subject's hearing.
Output transducer 24 converts the amplified electrical signals representing the tympanic vibrations into mechanical vibrations for application to an inner ear 28 of the subject. The amplified electrical signals are converted into corresponding mechanical vibrations by a second piezoelectric element within output transducer 24 (also not shown). The vibrations are communicated to inner ear 28 by a mechanical connection between wire 25B and a stapes 34. Stapes 34 then transmits these vibrations to an oval window 36 of inner ear 28. In this manner, the vibrations are transmitted to a cochlea 40 of the subject. The connection between wire 25B and inner ear 28 can be made in a manner similar to techniques employed in reconstructive surgery using passive mechanical prosthetic devices, for example.
Implantable hearing devices such as those in Schaefer have been used with the ossicular chain intact. However, Schaefer prefers that the ossicular chain be broken to prevent positive feedback of the amplified vibrations to input transducer 22 (e.g., via the incus). A break would typically be effected by removing at least one of the component parts of the ossicular chain, typically the incus. It is desirable to maintain the malleus and stapes in normal anatomical position with muscle and tendon intact to maintain the subject's natural defense mechanism against acoustic trauma. The hearing aid then becomes an integral piece of the ossicular chain.
As noted, wires 25A and 25B are preferably made of a substantially rigid material such as stainless steel. Such materials are used in the interest of accurately transmitting the mechanical vibrations correspond to sound waves 32, thereby providing faithful reproduction of the sounds received at the eardrum. Schaefer discusses several methods of attaching the distal ends of these wires to the ossicles or other structures.
For an implantable hearing aid such as that described in Schaefer to provide acceptable fidelity over the long term, however, the tension on the wires used to input and output mechanical vibrations preferably remains constant. While a simple attachment, such as is described in Schaefer, may provide acceptable performance in the near term, aging of the various auditory structures involved may cause these attachments to loosen or otherwise cause wires 25A and 25B to lose tension, impairing the hearing aid's performance.